Fire alarm systems used in buildings and such are designed to save lives and include a number of components and devices such as smoke and heat sensors, audible and visible indicators, and a fire-fighter's telephone system.
Additional equipment used by firefighters are two-way radios that fire personnel routinely use for communications. However, today's large modern buildings challenge the operation of these two-way radios. In order to provide fire personnel with reliable communication throughout the building, a dedicated wired firefighter's telephone system is commonly installed as part of the fire alarm system. The fire fighter's system consists of dedicated telephone stations strategically located in corridors, lobbies, mechanical control centers, stairwells, and other locations important to the fire fighting effort.
There are generally two types of telephone stations: 1) one that utilizes a hook-type cradle switch that signals the receiving end at the fire alarm control panel that an off-hook condition exists indicating a call-in is being requested at the outlying location, and 2) one that utilizes a portable plug-in handset that the fire personnel carry out to the outlying location and plugs into a wall-mounted receptacle that signals the receiving end at the fire alarm control panel that a call-in is being requested at the outlying location.
In either case, the fire fighter's telephones activate audible and visible signals at the fire alarm control panel indicating the outlying fire personnel is requesting communications with the fire alarm control panel operator. By answering the call-in request at the fire alarm control panel, the remote location is connected and communications may commence. Additional incoming calls from other fire fighters' telephones may also be selected at the fire alarm control panel similar to a party-line configuration.
Referring to FIG. 1, a conventional firefighter's telephone system 100 includes a microphone 105 and receiver 110 electrically connected to an R1 resister 120 and a C1 capacitor 125 as shown. A non-polarized telephone line 115 is generally utilized. Conventional firefighter's telephone systems 100 provide an additional feature not available on conventional telephone systems in that wiring supervision is provided so as to ensure the wiring between the outlying telephone station and fire alarm control panel is in tact and available during an emergency situation.
Traditionally, it has only been required to monitor the integrity for an open wiring condition between the firefighter's telephone station and the fire alarm control panel. An open wire will notify building management of the trouble condition and will limit the operational capability of the system until the fault is corrected.
In addition, it has not been convention to monitor the integrity of the wiring connection between the firefighter's telephone station and the fire alarm control panel for a shorted wiring condition because of the difficulty in distinguishing between the shorted wiring and the low impedance that normally results from an off-hook condition.